The electrodeposition of aqueous cationic resin compositions onto conductive substrates is well known in the art. It is standard operating practice among American automobile manufacturers to coat automobile frames and sheet metal with an anti-corrosive, electrodeposited cathodic resin composition which is cured to a hard, durable protective coating.
In a typical electrodeposition process an aqueous bath is prepared from a principal emulsion and a pigment paste. The principal emulsion typically comprises an aqueous epoxy-amine resin adduct which has been salted with an acid to solubilize the adduct and mixed with a cross-linking agent. Typical cross-linking agents include the blocked polyisocyanates. The pigment paste typically comprises a mixture of an epoxy-amine adduct which has been salted with an acid and a pigment which are ground together to form a pigment paste. The pigment paste is mixed with the principal emulsion and distilled water at the coating site to form an aqueous coating bath having the desired solids concentration. The aqueous coating bath is typically contained within an insulated tank having sufficient capacity to completely immerse any objects that will be coated therein. The tank contains an anode which is connected to a DC circuit. Additives conventional in the art may be added to the bath to improve coating characteristics.
An article which is to be coated typically comprises an electrically conductive material. The article is connected to a direct current circuit to act as a cathode. When the object is immersed in the coating bath contained in the coating tank, a flow of direct current electricity across the object causes the principal emulsion and pigment paste to be deposited on the surfaces of the coated article. The article is typically removed from the bath when the desired thickness of film has been deposited, then the article is optionally washed with distilled water. The article and deposited film are then typically moved to an oven where the film is cured to a smooth, hard, durable cross-linked coating.
Cathodic electrodepositable amine-epoxy resin adduct compositions, methods of manufacturing these cathodic electrodepositable resin compositions, aqueous cationic electrodeposition baths and processes for the deposition of these resins from a coating bath onto a conductive object are disclosed in U.S. Pat. Nos. 3,984,299, 3,468,779, 4,116,900, 4,093,594, 4,137,140, 4,104,147, 4,225,478, 4,419,467, and 4,432,850, 4,575,523, and 4,575,524.
Cathodic electrodepositable resin coatings provide a metal substrate with a superior corrosion-resistant primer coating. It is known that cationic resin compositions provide superior protection to a steel substrate than anodic resin compositions. In the automotive industry, these coatings are typically overcoated with a multi-layer coating such as a colored base coat and a clear top coat.
Although the electrodepositable cationic epoxy-amine resin adduct compositions are known to provide durable, smooth, corrosion resistant, hard films, it is also known that the hardness of epoxy-amine resin adduct films has several disadvantages. One disadvantage is that sudden impacts, wherein the underlying substrate is deformed, tend to cause the cured electrodeposited film along with any top coats to break away from the substrate. This is undesirable since many coated objects are subjected during the course of their typical usage to situations wherein sudden impacts with another object having sufficient momentum and/or mass result in dents in the coated object. The lifting of the coating from the dented portion of the coated object is undesirable since it is not always possible to immediately repair the damaged section of the coating and the underlying substrate is consequently subjected to corrosion for extended periods of time. In addition, automobile bodies are constantly exposed to high velocity flying debris such as stones, pebbles, etc. The collisions of the pebbles and debris with the coated substrate, depending on the mass, size and shape of the pebbles and the angle of impact, and the velocity typically causes paint to chip away at the point of impact between the pebble or stone and the coated automotive body, thereby exposing the underlying substrate.
Film forming compositions comprising polyurethane resin dispersions are known but these compositions produce cured films which are too soft for use as a primer and the films do not have the corrosion resistance required for a primer coating. In addition, these compositions are known to be available only for anodic electrodeposition.
Therefore what is needed in this art is a cationic electrodepositable resin composition which produces coatings having improved flexibility, improved impact-resistance and resistance to chipping along with smoothness, anti-corrosivity, and durability.